1. Field of the Invention
The present invention concerns a process and catalyst system for reduction of nitrogen oxides from exhaust gases using an oxygen-containing hydrocarbon reducing agent, such as dimethyl ether (DME). In particular the invention relates to dual-bed catalyst system for reduction of nitrogen oxides using an oxygen-containing hydrocarbon reducing agent in which the first catalyst bed contains alumina and the second catalyst bed contains indium supported on alumina. The process and catalyst system is suitable for use in the reduction of nitrogen oxides in stationary applications, such as reduction of nitrogen oxides from exhaust gases in power plants. More particularly, the process and catalyst system is used for reduction of nitrogen oxides in automotive applications, such as in lean-burn internal combustion engines.
2. Description of the Related Art
The emission of nitrogen oxides by exhaust gases in stationary and automotive applications has long been a major environmental issue and it is steadily subjected to more stringent environmental regulations. The harmful effects of nitrogen oxides (NOx) are well known and therefore intensive research is being conducted to find processes and catalyst systems which are able to cope with stricter environmental regulations. In conventional stationary NOx reduction systems, such as in processes for the reduction of NOx to nitrogen (N2) from exhaust gases of power plants, ammonia is used as reducing agent in selective catalytic reduction (SCR). The use of ammonia is however becoming less and less attractive, as environmental regulations are also pushing down the permissible levels of ammonia emissions. Therefore, the use of oxygen containing hydrocarbons such as dimethyl ether (DME) as reducing agent instead of ammonia is becoming attractive, although operation is normally restricted to a narrow temperature range. It would thus be desirable to be able to use such hydrocarbon reducing agents without impairing the catalyst activity towards NOx reduction to nitrogen and at a wide range of temperatures.
In the automotive industry engine manufacturers are also faced with the task of providing systems for NOx reduction in lean combustion engines. However, it has been difficult to come up with a process and catalyst system which not only removes NOx properly, i.e. provides high NOx conversion, but also operates at a wide temperature window in the presence of an oxygen containing hydrocarbon as reducing agent.
U.S. Pat. No. 5,336,476 discloses a process for reduction of NOx to nitrogen in which exhaust gas is contacted with a reducing catalyst that may be in the form of acidic metal oxides such as alumina, titanium oxide, zirconium oxides and mixtures thereof, and in the presence of an oxygen-containing organic compound such as dimethyl ether. The exhaust gas may be subsequently passed through an oxidizing catalyst containing noble metals, base metals or perovskite oxides on a carrier such as active alumina, silica or zirconia.
US 2007/0092421 describes a catalyst system for NOx reduction in the presence of an organic reducing agent in which a first zone comprises a catalyst support together with a catalytic metal comprising gallium and at least one promoting metal selected from the group of silver, gold, vanadium, zinc, tin, bismuth, cobalt, molybdenum, tungsten, indium and mixtures thereof. In a second zone subsequent the first zone the catalyst system comprises a catalyst support and a catalytic metal selected from the group of indium, copper, manganese, tungsten, molybdenum, titanium, vanadium, iron, cerium and mixtures thereof. The catalyst support in either zone comprises at least one of alumina, titania, zirconia, ceria, silicon carbide and mixtures thereof. The reducing agent includes alcohols, ethers such as dimethyl ether (DME), esters and others.
U.S. Pat. No. 6,703,343 discloses a catalyst for lean NOx exhaust comprising a specially prepared metal oxide catalyst support. This citation mentions independently the use of alumina or indium on alumina as a catalyst suitable for lean NOx reactions at high temperature in the presence of propene as reducing agent.